In recent years, there has been a growing demand for composite wafers represented by silicon on insulator (SOI) wafers. Of the SOI wafers, “silicon on quartz (SOQ)” and “silicon on sapphire (SOS)” have attracted attention as wafers comprising insulating transparent wafers as handle wafers.
SOQ wafers are expected to be applied to optoelectronics utilizing high transparency of quartz, or high-frequency devices utilizing low dielectric loss of quartz. SOS wafers are expected to be applied to high-frequency devices which involve heat generation, because handle wafers comprise sapphire and thus have not only high transparency and low dielectric loss but also high thermal conductivity which is unattainable by glass or quartz.
Such a composite wafer is typically produced by bonding two wafers (a donor wafer and a handle wafer). An SOS wafer can also be obtained by epitaxially growing silicon directly on r-plane of sapphire. Its crystal quality, however, usually falls short of bulk silicon because the lattice constant differs between sapphire and silicon.
At present, the diameters of a silicon wafer and a sapphire wafer are mainly 6 to 12 inches and 2 to 4 inches, respectively. For example, in the case of producing a 2-inch SOS wafer, a 2-inch silicon wafer needs to be prepared. However, it is currently very difficult to acquire a 2-inch silicon wafer. In the field of silicon devices, both a trend toward larger diameter and a trend toward finer structure of wafer have been ongoing. Accordingly, when attempting to obtain a wafer of excellent quality, it is inevitable to select a silicon wafer of a large diameter.
Patent Document 1 describes use of a handle wafer and a donor wafer larger than the handle wafer to allow a transferred layer to have an increased area. In Patent Document 1, however, one donor wafer is bonded to one handle wafer, so that it has to be subjected to primary chamfering and secondary chamfering on both of the wafers.